This invention relates to pianos and has to do more particularly with an improved key frame for use in a grand piano wherein the key frame, keys, and action normally can be removed as a unit from the piano case so as to provide easy access to the parts when it becomes necessary to work on or to adjust any of them.
The key frame in a conventional piano is normally a relatively flexible structure formed from an assembly of wooden rails which define a supporting frame. Usually the key frame comprises a series of three parallel or nearly parallel wooden main rails interconnected by a series of crossrails located along the main rails at spaced apart intervals and arranged generally at right angles to the main rails. Each of the three main key frame rails is nearly as long as the width of the piano and somewhat longer than the total width of the array of keys. These main rails normally are referred to as the front rail, the balance rail and the back rail, named in the order of increasing distance from the front of the piano into which the key frame is placed. The main rails lie approximately at right angles to the lengths of the keys and each serves a particular purpose.
The front rail is located below and adjacent the fronts of the keys, lying beneath their playing surfaces where the front rail serves as a stop against which the keys can bottom when they are depressed to play the piano. The keys are guided in their up-and-down motion at the front rail by means of polished vertical pins which are driven into the front rail, usually one pin for each key. The pins project upwardly from the top of the front rail and mate with feltlined holes projecting upwardly from the bottoms of the keys to a depth which depends upon the up-and-down key position. These front rail pins locate the keys at the front of the piano and permit them to move up-and-down freely while restricting sidewise motion so as to prevent adjacent keys from contacting each other. Customarily a felt washer, known as a "punching", is placed over each pin so that, when the key bottoms, the felt "punchings" will lie between the bottoms of the keys and the key frame front rail to prevent undesirable noise. In addition, paper washers normally are placed under the felts in whatever number and thickness is required to regulate the extent of up-and-down key movement or "depth of touch" of the keys.
The balance or middle rail acts as a fulcrum for the keys. Each key is positioned both left and right and fore-and-aft at this fulcrum by another essentially vertically disposed pin driven into the balance rail. These pins are received in slot-like openings projecting upwardly from the bottoms of the keys so as to allow the keys to pivot fore-and-aft in the normal piano playing motion without permitting excessive side play or fore-and-aft sliding motion of the keys. At the balance rail the bottoms of the keys also rest on felt punchings placed over the balance rail pins; and paper punchings of various sizes and in the correct number are placed beneath the felt punchings to obtain accurate key leveling, i.e., to adjust the height of the playing surface of each key to be the same from key to key when the keys are at rest in their non-depressed or "key-up" position.
The third rail or so-called back rail functions as a resting point for the rear portion of each key, the back rail in combination with the height of the balance rail and number of balance rail punchings determines the angle of the keys and the elevation of the front portions of the keys when they are at rest. Normally the back rail is covered with a continuous strip of felt running lengthwise along the rail on which the rear undersurfaces of the keys rest when the key fronts are in their at rest or "key-up" positions. In a conventional piano there usually is no adjustment of key position made at the back rail.
In addition to mounting the keys in the manner described, the key frame also mounts the so-called action or mechanism by means of which the motion of the keys during the playing of the piano is translated into the motion of a set of felt covered hammers which strike the piano strings, exciting them to vibrate. The nature and exact arrangement of the action which, for each key, comprises a mechanical system of levers and pivots, is not important to the present invention and there are numerous variations which may be utilized in connection with the invention. Generally speaking, however, the action consists of a framework comprising two rails disposed generally parallel to the front width of the piano, the first comprising an upper or hammer rail on which the hammer assemblies are mounted, and the second comprising a lower or wippen rail on which the wippen assemblies are mounted, the wippen assemblies serving to actuate the hammers when the keys are stuck. The hammer and wippen rails are mounted in spaced apart relationship by five or six so-called action brackets mounted on the key frame at spaced apart intervals, the action brackets usually being secured to the crossrails of the key frame, which are preferably spaced apart at intervals of several centimeters to coincide with the location of the action brackets. The action brackets are provided with foot-like flanged projections on their bottom surfaces which are drilled to accept mounting screws by means of which the brackets are secured to the key frame structure with the hammer and wippen rails positioned so that each hammer and wippen assembly is in proper position to cooperate with the appropriate key on the key frame to strike the correct strings for a given note. It will be understood that there are various ancillary weighting and regulating adjustments to make the parts function properly and have the correct touch weight, but these matters are familiar to the piano builder and, as such, do not constitute a part of the present invention.
A complete key frame-key action assembly may weigh on the order of 30 kilograms or more and hence must have substantial support when it is placed in the piano case. In order to support this rather massive assembly, the conventional grand piano case is provided with a supporting shelf called the key bed, which is a heavy and expensive assembly normally consisting of several interconnected wooden rails extending across the front of the piano case to provide a supporting shelf for the key frame. The key bed so-formed is rigidly connected to the piano case so as to become essentially an integral part of the piano case. Because of mechanical coupling existing between the key bed and the piano soundboard via the various parts of the piano case, some portion of any vibrations originating at the key bed will be transmitted through the frame of the piano to the soundboard to produce undesirable knocking or extraneous noise components which may interfere with or detract from the tonal beauty of the instrument when it is played. Consequently, when the key frame-key action assembly is installed in the piano case the rails of the key frame must be in intimate contact with the key bed which supports the entire weight of the assembly. If such intimate contact is not achieved, as for example where small clearances exist between the key frame and the key bed, the key frame will impact against the key bed when the piano is played, and an undesirable knock in the tone may result. In this connection, it must be understood that in a grand piano it is necessary to shift the entire key frame-key action assembly laterally with respect to the strings in order to provide for the use of the una-corda pedal. Conventional musical notations require the use of such a pedal to move the action so that not all of the strings of a given note are struck by the piano hammers. This is done in order to produce a certain tonal effect; and in grand pianos the accepted practice is for the entire key frame to be shifted by sliding it relative to the key bed. Consequently the key frame must be free to shift on the key bed and hence the key frame cannot be rigidly fastened to the key bed by screws or bolts which would otherwise prevent slight changes in the shape of the key frame from producing knocks due to the resulting gaps.
In order to prevent undesirable knocking the key bed must be carefully constructed of properly dried wood so that any tendency to warp will be minimized, and after attachment of the key bed to the piano case, it must be carefully planned so as to be flat and level. Similarly, the key frame heretofore has been made of properly dried, unwarped, straight wooden rails which must be carefully fitted to the key bed. After the initial mating of the key frame and the key bed, small amounts of wood must be carefully removed from the key frame front rail by hand-sanding in order to secure an absolutely perfect fit so as to prevent knocks or thumps at the front rail when the piano is played. In order to reduce the amount of effort involved in fitting the parts together, the bottoms of the front and back key frame rails usually are planed or machined to have a slight bevel so that only one edge of each rail will be in contact with the key bed. In addition, the balance rail normally does not contact the key bed directly but rather is supported at intervals by adjustable support screws installed in the balance rail. Such screws are necessary because without them it would be extremely difficult, if not impossible, to achieve a satisfactory and knockless fit between the key frame and the key bed. Even if a satisfactory fit can be initially achieved, it is very difficult to maintain such fit over a long period of time in use. Changes in temperature and relative humidity can affect the shape of the parts and cause warping or changes in the dimensions of the parts which are sufficient to cause a knock to appear even when the key frame to key bed fit is initially correct. In addition, even when the fit is correct, the relatively large area of intimate contact between the key frame and key bed may transmit too much key-bottoming impact noise to the piano soundboard.
With the foregoing background in mind, the purpose of the present invention is to provide a novel key frame-key bed construction which effectively eliminates the difficulties inherent in conventional wooden constructions.